Process for reducing the arsenic content of gaseous hydrocarbon streams by the use of selective activated carbon

ABSTRACT

A process for reducing the arsenic content of a gaseous hydrocarbon stream by contacting the stream with a sorbent comprising a specially defined metals content activated carbon prepared from a bituminous coal.

Unite States P tent Stahfeld Sept. 3, 1974 [54] PROCESS FOR REDUCING THE ARSENIC 2,511,288 6/1935 Morrell et al 423/239 CONTENT O GASEOUS HYDROCARBON 2,513,508 7/1950 Morrell et a1......... 252/431 STREAMS BY THE USE OF SELECTIVE :[ppell 208/88 attox et al 208/251 R ACTIVATED CARBON 2,911,353 11/1959 Watts et al 208/88 75 Inventor; Donald L s f l Glenshaw, 2,939,833 6/1960 Wankat 208/91 3,223,748 12/1965 Bohner 208/307 [73] Assignee: Gulf Research & Development 3,542,669 11/1970 Defeo 208/307 Company, Pittsburgh, Pa, 3,546,103 12/1970 Hammer et a1 208/253 3,617,481 11 1971 H 1.. 208 2 22 Filed: July 18, 1973 I a I l 53 [21] Appl. No.: 380,208 Primary Examiner-Delbert E. Gantz Assistant ExaminerJuanita M. Nelson [52] US. Cl 208/307, 208/88, 208/253,

423/438, 252/431, 260/677 A [57 ABSTRACT [51] Int. Cl C10g 17/00, ClOg17/O2 [58] Field of Search 208/88, 251, 307, 253; Prom:SS educmg the arsemc'comem a 1 I 423/438 ous hydrocarbon stream by contacting the stream with a sorbent comprising a specially defined metals con- [56] References Cited 1 tent activated carbon prepared from a bituminous UNITED STATES PATENTS 1,520,437 12/1924 Pipkin ,423/393 10 Claims, 1 Drawing figure HYDFOGEN FLUID CATALYTIC 6 ig/1%? f I "231? cRAcKER V t 4 GASOLINE I6 34 ETHYLENE ARSENIC REMOVAL e -18 UNIT V 30 42 HEAVIER -29 PRODUCTS ABSORBER DRYING ssc'non g/ERY. c

, 12 2o SYSTEDM 24 3 PROCESS FOR REDUCING TI-IE ARSENIC CONTENT OF GASEOUS HYDROCARBON STREAMS BY THE USE OF SELECTIVE ACTIVATED CARBON This invention relates to the removal of arsenic from gaseous streams, and more particularly to a-process for reducing the arsenic content of a gaseous hydrocarbon stream, by the use of a specially defined metals content activated carbon derived from bituminous coal.

BACKGROUND OF THE INVENTION Catalytic cracking is one of the principal methods for refining petroleum fractions to recover more valuable hydrocarbon products such as gasoline. The unit in which the cracking operation takes place generally employs a fluidized bed and thus is termed a fluid catalytic cracking (FCC) unit. A variety of lower boiling prod ucts in gaseous form are discharged from the FCC unit and these are usually further treated to recover separate hydrocarbon fractions, e.g. ethylene. This further treatment of FCC vapors may, as in the case of the hydrogenation of acetylene, involve the use of a noble metal catalyst. As is well known, noble metal catalysts are rapidly deactivated by feedstock impurities such as arsenic. It thus becomes desirable to reduce the arsenic content of the FCC gases to the lowest possible level before subjecting them to further treatment.

It should be noted that the exact form in which arsenic is present in FCC gases is difficult to determine. It is known, however, that FCC gasesin which arsenic can be detected cause the aforesaid deleterious effects upon a noble metal catalyst. Although it is believed that a major portion of the arsenic contained in the gases is present as arsine (ASHa), the term arsenic as used herein is intended to include arsenic in any combined gaseous form.

DESCRIPTION OF THE PRIOR ART It is well known that arsenic in gaseous form is a highly toxic substance. Workers in the gas mask art have suggested the use of activated charcoal impregnated with a metal or metal oxide such as copper or copper oxide as a material through which air (or other oxygen-containing gases) may be passed for the removal of arsenic. Exemplary of these proposals are U.S. Pat. Nos. 1,520,437; 2,511,288; and 2,513,508.

It has been found more recently that the presence of arsenic in gasolines which are treated by contact with a noble metal containing catalyst causes a permanent deactivation of the catalyst. Suggestions have been made in the art to pretreat petroleum fractions to remove arsenic by use of a wide range of materials such as a lignitebased activated carbon (U.S. Pat. No. 3,542,669); silica gel impregnated with sulfuric acid (U.S. Pat. NO. 3,093,574); aluminum silicate (U.S. Pat. No. 2,939,833); or a salt of a metal not higher than copper in the electromotive series of metals (U.S. Pat. No. 2,781,297).

SUMMARY OF THE INVENTION It has now been discovered that a specially defined activated carbon derived from ya bituminous coal will directly remove arsenic from gaseous hydrocarbon streams. For purposes of this application, the activated carbon material will be termed a sorbent, although that term is not intended to suggest that the arsenic resuch as H S since sulfur compounds may interfere with moval is accomplished solely by physical adsorption. While not wishing to be bound by any particular theory, it is believed that some chemical reaction is involved between the arsenic and the sorbent, but the exact nature of the reactions are not known. At a minimum, it is believed that the removal of arsenic is accomplished, at least in part, by chemisorption; that is, the arsenic forms bonds with the surface atoms of the sorbent that are of comparable strength with ordinary chemical bonds and stronger than the bonds formed in physical adsorption. I

Particularly surprising is the fact that other forms of activated carbon, such as those derived from lignite, are very inferior in their activity for the removal of arsenic from gaseous hydrocarbon streams. It should be noted that while many materials will function to remove arsenic from admixture with inert gases such as argon, and such materials remain active for reasonable loadings of arsenic, most of these materials fail quickly in the removal of arsenic from light hydrocarbon gases such'as ethane-ethylene streams. In this context, the term breakthrough means the passage of arsenic be- .yond or downstream of the substance intended to remove it and is usually expressed as a percentage of the arsenic not removed in relation to the arsenic content of the charge stock. I

The charge stock for treatment in accordance with the invention is a gaseous hydrocarbon feedstream wherein the hydrocarbons preferably have from one to four carbon atoms per molecule and which feedstream contains arsenic as an impurity in an amount from about 20 ppb to about 20 ppm or more. In this application the term ppb means parts per billion, and ppm means parts per million, and such parts are parts by volume unless otherwise indicated. Particularly preferred for treatment by the process of this invention are those light unsaturated hydrocarbon gases obtained by the catalytic cracking of heavier petroleum hydrocarbons such as gas oils or the thermal cracking of propane streams to produce ethylene-ethane streams. The light gases from an FCC unit have been found to contain small concentrations of arsenic, even though arsine, for example, is known to decompose at about 450F. (232C.) and the temperatures in an FCC unit are known to reach over 900F. (482C). There is probably insufficient contact time in an FCC unit to decompose the arsine, or perhaps the arsine decomposes and reforms on cooling. For whatever reason, the present invention is applicable to those light hydrocarbon streams, especially olefinic streams, containing more than 20 ppb of arsenic, regardless of the source of thearsenic.

Preferably the charge stock of this invention is substantially dry, and by this is meant that the charge stock contains less than 10 ppm of water. While the use of a dry charge stock is not essential, the absence of water permits the operation to continue for a much longer time before breakthrough of the arsine. If water is present in the gaseous hydrocarbon stream, such as the gaseous hydrocarbon stream obtained as a product from an FCC unit, the water can be removed by any suitable procedure well known in the art such as by the use of molecular sieves or other drying agents. Similarly, the charge stock is preferably free of sulfur compounds the removal of arsines from gaseous hydrocarbon charge stocks and reduce the loading ofthe sorbent before breakthrough. Again the manner of removing sulfur compounds from the charge stock in the preferred embodiment may be by any of the methods well known in the art, and such methods include, for example, the use of liquid solutions of amines or the use of caustic solutions, e.g., sodium hydroxide solution.

The process of the invention will now be further described by reference to the attached FIGURE. Referring to the FIGURE, the petroleum charge for catalytic cracking enters through line 2 into FCC unit 4 where it is converted under usual catalytic cracking conditions to'a variety of lower boiling products, including gasoline type products. Gasoline is removed from FCC unit 4 through line 6. The other gaseous products of the cracking process, which products are of primary concern here, are removed from FCC unit 4 through line 8 and enter an absorber section 10. Absorber section 10 normally consists of several component units .(not shown) such as an amine absorber, a knockout drum to remove any entrained liquids from the gaseous products, and a heater to insure that the gases remain in the vapor phase. The FCC gases exiting from the heater unit of absorber section 10 have the typical composition shown in the following Table I:

TABLE I Component Nitrogen Hydrogen Methane Ethylene Ethane Propylene Propane Butenes Butanes Pentenes Pentanes Hexanes Carbon Monoxide 250 to 400 psig (17 to 27 atmospheres), more usually at a pressure from 290 to 360 psig (20 to 24.5 atmospheres). The increased pressures are those normally employed in the FCC unit and are used to propel the gases through the various units in the recovery train.

Light hydrocarbon gases such as ethane and propane are fed through line 14 into pyrolysis furnace 16 for the purpose of cracking the ethane and propane to produce ethylene. After recovery of liquid product (not shown) from pyrolysis furnace 16, the gaseous products are passed through line 18 where they are combined with the products in line 12 from absorber section 10 and are passed through line 20 into system 22 which consists of a number of units, not individually shown, for the purpose of drying and recovering various hydrocarbon fractions. A C fraction, for example, can be removed through line 24, and a C fraction through line 26. The stream of most present interest and of greatest volume is the C stream containing small amounts of acetylene, which stream is shown in the FIGURE as being removed from systems 22 through line 28. A typical composition of this C stream is shown on Table 11 below.

TABLE II V01. V01. Component Range Acetylene (C l-I 0.50 0.1 1.0 Ethylene (C H,) 65.0 Ethane (C H l 34.5 45 25 The ethane-ethylene stream in line 28 is of the greatest interest commercially, and it is this stream which is passed into arsenic removal unit 30.

The function of arsenic removal unit 30 is to reduce the concentration of arsenic in the ethane-ethylene gases from a concentration in excess of 20 ppb to a concentration at the outlet of less than 20 ppb. The concentration of arsenic in the ethane-ethylene gases is usually on the order of 100 to 2,000 ppb but can be as high as 20 ppm or more. Preferably, the arsenic content of the gases leaving arsenic removal unit 30 is less than 10 ppb and is more preferably less than 2 ppb.

The type of sorbent material employed in arsenic removal unit 30 is an important feature of this invention and will be discussed in detail hereinbelow. Suffice it to say here that the sorbent material comprises a special metals content activated carbon derived from a bituminous coal.

The temperatures to be employed in the arsenic removal unit 30 can suitably be from 50 to 200F. (10 to 93C.), are usually from to 150F. (27 to 55C.), and are preferably from to F. (32 to 49C.). Temperatures below 50F. (10C.) are undesirable because of increased costs. Similarly temperatures above the stated range are undesirable due to the increased expense of operating the process.

The pressure to be employed in arsenic removal unit 30 is suitably atmospheric pressure or below to 1,000 psig (69 atmospheres) or more. As noted above, FCC units typically operate to produce product gases at pressures of about 250 to 350 psig (18 to 25 atmospheres). The process of the invention will operate satisfactorily at atmospheric pressure, but since it is expensive to depressure the FCC absorber gases and repressurethe final products for transport through pipelines, it is desirable to operate the arsenic removal unit 30 at increased pressures of, say, 250 to 350 psig for transportation purposes.

Gaseous volume hourly space velocity (GVI-ISV) at standard conditions of temperature and pressure can suitably be from 1,000 to 20,000 v/v/hour, and is usually from 2,000 to 10,000 v/v/hour. The product is re moved from the arsenic removal unit 30 through line 32, and these gases, free of arsine but containing acetylene, are passed into an acetylene converter 34. The acetylene content isproduced in the pyrolysis furnace l6. Hydrogen enters acetylene converter 34 by means of line 35.

Acetylene converter 34 may contain a catalyst which is sensitive to poisoning by even minute quantities of arsenic, and thus it is one of the main objectives of the present invention to protect the catalyst in the acetylene converter 34 from permanent deactivation by arsenic. Catalysts which are particularly susceptible to arsenic poisoning are those containing the noble metals such as platinum and palladium.

Hydrogenation conditions are, of course, employed in acetylene converter 34, and such conditions are well known to workers skilled in the art. The C stream, substantially free of acetylene, is then taken from acetylene converter 34 through line 36 to a distillation zone 38 where ethylene is removed through line 40 and heavier products may be suitably removed through line 42. The heavier products may be recycled as feed to pyrolysis furnace 16, if desired.

It should be noted here that the same benefits would accrue for any arsenic susceptible catalysts used in the hydrogenation of the propadiene in the C stream in line 24 or the butadiene in the C stream in line 26. Thus the olefinic streams-in line 24 or in line 26 could be passed into an arsenic removal unit similar to unit 30.

The sorbent material for use in the process of this invention is a specially defined metal containingactivated carbon derived from a bituminous coal.

The preparation of activated carbon from various coal sources has been known for extended periods of time. Exemplary methods are described in the Kirk- Othmer- Encyclopedia, Second Edition, Vol. 4, pages 149 to 156, and this description is incorporated herein by reference. Bituminous coals are similarly well known and are described, for example, in The Production-of Active Carbon from Bituminous Coal, J. G. King, D. MacDougall and H. Gilmour: Technical Paper No. 47, Dept. Scientific and Industrial Research, Fuel Research, His Majesty s Stationery Office, London, England (1938); and this description is incorporated herein by reference.

Coals naturally contain various metals in differing concentrations. It has now been found that a certain activated carbon prepared from a bituminous coal has unusually superior ability for the removal of arsenic from light olefinic gas streams such as the combined C stream from an FCC unit and an ethylene pyrolysis furnace.

Various activated carbons were tried for this purpose, and it was found from a comparison of the properties of the activated carbons that those activated carbons derived from bituminous coal and containing certain concentrations of cobalt, molybdenum, nickel and vanadium were surprisingly active for arsenic removal. The amounts of these metals per 100 cubic feet of activated carbon are suitably from 0.1 to 1.0 pound of cobalt, preferably 0.1 to 0.3 pound; 0.5 to 2.0 pounds of molybdenum, preferably 1.25 to 1.75 pounds; 0.1 to 1.0 pound of nickel, preferably from 0.2 to 0.6 pound; and 0.5 to 5 pounds of vanadium, preferably from 1.5 to 2.5 pounds.

Furthermore, it is preferred that the activated carbon have'less than 1 pound of magnesium and 2 pounds of titanium per 100 cubic feet of the activated carbon. I

The surface areas of the activated carbons are high, usually on the order of 500 to 1,500 m /g.

The invention will be further described with reference to the following experimental work.

A series of runs was made with several different activated carbons to determine the effectiveness of these activated carbons for the removal of arsine from a given charge stock. In all of the runs, the experimental The reactor containing the activated carbon sorbent consisted of a inch I.D. stainless steel cylinder with a A; inch O.D. thermowell extending along its axis. The reactor was suitably heated. The temperature at the center of the activated carbon was measured by means of aniron-constantan thermocouple inserted into the thermowell. The test gas was introduced at the bottom of the reactor, passing through an approximately 6- inch-long bed of quartz chips, which served to preheat the gas stream.

The bed of activated carbon within the reactor was cal transmittance at 540 mm. wavelength was measured with a Bausch and Lomb Spectronic spectrophotometer. This optical transmittance was then plotted as a function of time. The numerical derivative of this curve was calculated to determine the rate of arsine breakthrough. The percent breakthrough figures given in the tables below represent the percentage of the arsenic not removed in relation to the arsenic content of the charge stock.

The results of a first series of runs is shown in Table III below:

TABLE III Removing Arsine with Activated Carbon Using 50 ppm by Weight of AsH; in N,

"Purchased from \Vestvaco known as Nuchar-WV-H (Tradename). Purchased from Atlas Chemical (30.. known as Darco (Tradename). Experimental high-ash carbon.

"Purchased from Calgon Corp. known as BPL (Tradename).

Referring to Table III it can be seen that the activated carbon used in Example 1 is surprisingly more active than any of the other activated carbons.

Tables IV and V below contain detailed analyses of the various activated carbons used in the'experirnents shown in Table III above.

TABLE IV Activated Carbon Ash Content/Analyses Carbon from Example from 1 2 3 4 Table 11! Density: lbs/ft 30 24 32 24.5 Wt Ash .85 14 16.9 6.74 Lbs.Ash/l00 ft 206 336 540 of carbon TABLE lV-Continued ume and pore area occupied by pores having a 7-10 A. v pore radius; and from 10 to 25 percent of their pore Activated Carbon Ash Content/Analyses volume and pore area occupied by pores having a 10-15 A. pore radius.

Example 5 In the run for this example, a slip stream of a C, concentrate from a commercial unit was passed through a bed of an activated carbon as used in Example 1 above at ambient temperature, 300 psig (20 atmospheres), and a GHSV of about 8,000. The charge had a compo- 3d V... WI c m m h .1 a t S r. X eA w E r mb 0 f m e 0.0 0 m a n 8 0d 6 g b e 0. r H 6 U C e C 6 0 h 0 H r. f. a a p th .m 6 p e e u 6 m b C O t .m m p X f. n em 6 6 n m dk c ni e a [k 3 X t e e VJH E a r p S 3 Mb u A w mm 4 h 3 m m s m .J .JJJJ .22 w w WM v %0000000000000 0026 6 w H 2.1 5 M w 4 J a p w m m s h 7 0 ma f r. O H p34 5 .13Afi m 2Jj-J895D5A 2. W700 000000000 240 k t 1 ew? m w .l t e m mm m H m w m .s .23 .s .ns .n m 0 m%0000000000000 247n s 1 5 0 n 4 8 50 1 2 o 3 o 0 s4 m l 1 a 11. s2 .s .nosA5 C W%0.l1 1.l.l.l1l1 3 87 d 4 55 e a a n x... a m I V w mo wwwummm .n w 24315110 41 393 3 c %000 000 L0 L1 24766 v A .l i A 5 5 .l 6 34 E w 2 4 0 l 84 9 8 3 .0. 4. .704 L a sJJzAAonezssnssnde 11 0%2l5 B V a 247 233332223347 0 T m .m w .JJ .JJ2 .J .350 .J2 7 M %00000000000000 2 0 5336 415.06 e 5 18 P H 9|. ssAssAssfinJsdflszsz w W% 00 0000 000 2572 5 8 .l- 6 85- 05 0 5 W. 1 .4 .2 JAAB h 02 00 00 012 P l 4 h p .u Q S n mm wit A. Eh W. M We aW W0O00O 0 2 5 5 5 50 mew ma ma a jjiism bim t uwm m m m m mmfl MGOOOOOOOOOOSOSOSOSO r m u umm m A A 5N5W98765443322 i i l w m w amummm mm mm 8 P gnlka a f V E 0 a ctid n RT P ooe i nna m A B CHLMMMNPSSSTV C Table 111 Lb.of metal/100 ft of carbon Aluminum Boron Calcium Chromium Cobalt Carbon from Example from x srisaslns ramqn 9" II 1',$ 'EI BErM. University (1943); S. Brunauer, Chap. I 1. pp. 365-413.

Resort may be had to such variations and modifications as fall within the spirit of the invention and the I claim:

1. A process for reducing the arsenic content of a gaseous hydrocarbon charge stock which comprises contacting the charge stock with an activated carbon derived from a bituminous coal and wherein said activated carbon per 100 cubic feet has from 0.1 to 1.0 pound of cobalt; 0.5 to 2.0 pounds of molybdenum; 0.1 to 1.0 pound of nickel; 0.5 to 5 pounds of vanadium.

2. A process in accordance with claim 1 wherein the charge stock comprises a gaseous olefin having from 2 to 4 carbon atoms per molecule and greater than 20 ppb of arsenic.

See Absorption of Gases 8t. Vapors, Vol. 1, Princeton Referring to Tables III, IV and V, it can be seen that carbons derived from bituminous coals per se are not equivalent (compare Examples 1 and 4) and that the 55 scope of the appended claims.

"Obtained using "abs orptomat'f instrument manufactured b Appears low compared to manufacturer's specification.

' superior results shown in Example 1 cannot be attributed to the ash content of the activated carbon. It is observed, however, that only the activated carbon of Example 1 contains cobalt, molybdenum, nickel and vanadium in the concentration ranges defined above. The lignite carbon (Example 2) and the experimental high ash carbon (Example 3) contain none of these metals, while the bituminous carbon of Example 4 has only small amounts of nickel and vanadium and no cobalt or molybdenum. Further, from the data in Table V, it appears the carbons useful in the process of this invention preferably have from 55 to percent of their pore vol- 3. A process in accordance with claim 2 wherein the charge stock is the combined C stream from an FCC cracker and an ethylene pyrolysis furnace.

4. A process in accordance with claim 3 wherein the charge stock contains less than ppm water.

5. A process in accordance with claim 4 wherein the activated carbon in addition has less than 1 pound of magnesium and 2 pounds of titanium per 100 cubic feet of activated carbon.

6. A process in accordance with claim 3 wherein the charge stock comprises from 55 to 75 volume percent ethane; to 45 volume percent ethylene; 0.1 to 1.0 volume percent acetylene.

7. A process according to claim 6 wherein the activated carbon has from 0.1 to 0.3 pound of cobalt; 1.25

to 1.75 pounds of molybdenum; 0.2 to 0.6 pound of nickel; and 1.5 to 2.5 pounds of vanadium.

8. A process in accordance with claim 7 wherein said contacting occurs at a temperature from 50 to F.

9. A process in accordance with claim 8 wherein the charge stock is substantially free of sulfur compounds.

to 15 A. pore radius. 

2. A process in accordance with claim 1 wherein the charge stock comprises a gaseous olefin having from 2 to 4 carbon atoms per molecule and greater than 20 ppb of arsenic.
 3. A process in accordance with claim 2 wherein the charge stock is the combined C2 stream from an FCC cracker and an ethylene pyrolysis furnace.
 4. A process in accordance with claim 3 wherein the charge stock contains less than 10 ppm water.
 5. A process in Accordance with claim 4 wherein the activated carbon in addition has less than 1 pound of magnesium and 2 pounds of titanium per 100 cubic feet of activated carbon.
 6. A process in accordance with claim 3 wherein the charge stock comprises from 55 to 75 volume percent ethane; 25 to 45 volume percent ethylene; 0.1 to 1.0 volume percent acetylene.
 7. A process according to claim 6 wherein the activated carbon has from 0.1 to 0.3 pound of cobalt; 1.25 to 1.75 pounds of molybdenum; 0.2 to 0.6 pound of nickel; and 1.5 to 2.5 pounds of vanadium.
 8. A process in accordance with claim 7 wherein said contacting occurs at a temperature from 50* to 120*F.
 9. A process in accordance with claim 8 wherein the charge stock is substantially free of sulfur compounds.
 10. A process according to claim 8 wherein the activated carbon has from 55 to 85 percent of its pore volume and pore area occupied by pores having a 7 to 10 A. pore radius and from 10 to 25 percent of its pore volume and pore area occupied by pores having a 10 to 15 A. pore radius. 